(1) Field of the Invention
The present invention relates to a curved lamp manufacturing method for manufacturing a curved lamp by curving a straight glass tube, a curved lamp, and a backlight unit.
(2) Description of the Related Art
Cold-cathode fluorescent lamps (hereinafter referred to as fluorescent lamps) used in backlight units for liquid crystal display screens are one example of conventional curved lamps. The fluorescent lamps are in the shape of character “U” or “L”. When they are used in a direct-below-type backlight unit, a plurality of fluorescent lamps in the shape of character “U” are arranged at the back of the screen. When they are used in an edge-light-type backlight unit, the fluorescent lamps in the shape of character “L” are attached to the rim of a rectangular optical waveguide.
In the fluorescent lamps, it is required that the bending radius (curvature radius) of the curved portion is as small as possible (as close to the right angle as possible). The following is the reason for it. In the case of the direct-below type backlight unit, the larger the curvature radius of the curved portion is, the less the number of fluorescent lamps to be arranged is. If a small number of fluorescent lamps are used, only low brightness is provided by the backlight unit. In the case of the edge-light type, the larger the curvature radius at the curved portion is, the greater the distance between the fluorescent lamps and the optical waveguide is. If there is a great distance between the fluorescent lamps and the optical waveguide, only a small amount of light enters the optical waveguide, the light distribution characteristics is deteriorated, and the backlight unit becomes large in size.
In the direct-below type, to average the light distributed to the screen, for example, the curved portion in the shape of character “U” is arranged at a position outside the screen display area. The larger the curvature radius of the curved portion is, the greater the length of the curved portion is. As a result, if the curvature radius of the curved portion is increased, the curved portion outside the screen display area is extended as much, and a large amount of light, among the light emitted from the lamp, is wasted (not illuminating the screen). Also, this makes the outer frame of the screen larger in width, making the monitor apparatus itself larger in size.
In manufacturing such a fluorescent lamp, first a straight lamp (hereinafter referred to as a straight-tube lamp) is manufactured by applying a fluorescent substance onto the inner surface of a straight glass tube, attaching electrodes to both ends of the straight glass tube, and filling the glass tube with mercury, a rare gas or the like. The manufactured straight-tube lamp is then curved into the shape of character “U” or “L”. It should be noted here that the pressure inside the glass tube is kept to be lower than the atmospheric pressure.
The curving of the straight-tube lamp into the shape of character “U” or “L” is performed after the planned curved portion of the straight-tube lamp is heated and softened. As one example of the method for heating and softening the straight-tube lamp, Japanese Laid-Open Patent Application No. 6-243782 discloses use of a coil that is formed by winding a heating lead wire made of a nichrome wire or a Kanthal wire (hereinafter, the coil of the heating lead wire is referred to as a coil heater).
More specifically, while being held to extend in a horizontal direction, the straight-tube lamp is passed through the coil heater. One end of the straight-tube lamp is then fixed by a fixed chuck and the other end is held by a roller guide chuck in a manner that the roller guide chuck can move in the longitudinal direction of the straight-tube lamp. When the straight-tube lamp is in the above-mentioned state, the coil heater is electrified to heat and soften the planned curved portion, while rotating the fixed chuck along a horizontal plane around a predetermined axis.
Meanwhile, in recent years, as display panels of the liquid crystal display apparatuses have become thinner, glass tubes used in such displays have become narrower. Accordingly, glass tubes as narrow as 1-8 mm in the outer diameter have been used.
However, the above-described conventional method has a problem in manufacturing such narrow lamps. That is to say, the conventional method cannot be used to curve such narrow glass tubes, or such narrow lamps are manufactured at extremely low manufacturing efficiency. This is because the curving process makes the outer side of the curved portion thinner and weaker. As a result, when such a narrow glass tube is partially curved, the curved portion becomes weak, and by the negative pressure inside the glass tube and the atmospheric pressure, the weak portion is collapsed. In the conventional method, to prevent such collapses, a predetermined level of the curvature radius is required. This prevents the backlight unit from being made compact, and also prevents the improvement in the brightness and light distribution.